Ductile niobium base alloy



United States Patent 3,022,163 DUCTILE NIOBIUM BASE ALLOY Neil M. Lottridge, Jr., Warren, and Douglas G. McCullough, Rochester, Mich, assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Filed May 18, 1959, Ser. No. 814,100 Claims. (Cl. 75-174) This invention relates to a niobium base alloy having outstanding ductility, as well as excellent fabricab lity and high temperature oxidation resistance. It pertains particularly to a refractory metal alloy of this type which is designed for gas turbine engine components which reach temperatures up to 2000 F. under conditions where except onally high strength is not a primary requirement.

The nickel base alloy and cobalt base alloy components commonly used today in gas turbine engines for aircraft normally have maximum service temperatures of approximately 1800 F. to 1900 F. This limitation necessarily restricts the performance and efiiciency of these engines. Refractory metals, such as nobium, tungsten, molybdenum and chromium, have satisfactory high melting temperatures and sufiicient potential availability to warrant investigation for turbine engine applications. However, each of these metals exhibits poor ox dation resistance at temperatures of 2000 F. or above. Such metals are therefore unsatisfactory when exposed to extremely hot oxidizing gases, and prior attempts to correct this deficiency by adding small amounts of various alloying elements were generally unsuccessful. Some of the resultant alloys still did not possess'adequate oxidation resistance at the very high temperatures under consideration, while others were excess vely brittle.

Consequently, a principal object of the present invention is to provide a refractory alloy whi-h can be em ployed as a fabricated component of a turbine engine at temperatures up to 2000 F. because of its oxidation resistance at such temperatures and which possesses exceptional ductlity. Such an alloy also must have the necessary fabricability and a melting point of at least 3000 F.

These and other objects are attained in accordance with this invention with a refractory alloy comprising about to titanium, 9% to 11% molybdenum, 4% to 8% chromium, 1% to 5% zirconium and the balance (56% to 71%) substantially all niobium. Optimum results are obtainable when the alloy contains approximately 7% to 8% chromium and the zirconium content is maintained within the range of 3% to 5%.

The molybdenum serves to materially increase the oxidation resistance of the niobium base alloy and also contributes to its hot strength. When the molybdenum content is lower than about 9% or higher than approximately 11%, the oxidation resistance of the alloy is no ticeably reduced.

Titanium further improves resistance to high temperature oxidation, although it is also necessary in order to ,provide the niobium base alloy with the desired amount of ductility. If the amount of titanium present is less than about 15%, the oxidation resistance of the alloy is inadequate. On the other hand, when more than 20% titanium is present the alloy becomes soft and weak.

Likewise, chromium contributes to the oxidation resistance of the n obium base alloy as well as increasing its hot strength. A chromium content of at least 4% is necessary for oxidation resistance, but if more than approximately 8% chromium is included, the microstructure of the alloy becomes unstable and the alloy is relatively brittle. The addition of about 7% to 8% chromium is desirable to achieve maximum oxidation resis'tance.

The zirconium measurably increases the strength of the niobium base alloy, and at least 1% zirconium is necessary to maintain this property at an adequate level because of the high titanium content of the alloy. It is preerable to use 3% to 5% zirconium in order to meet strength requirements, particularly when the t'tanium content is near the upper end of the aforementioned range. If more than 5% zirconium is present, the niobium base alloy becomes susceptible to atmospheric contamination to an excessive extent. The alloy can tolerate this relatively high zirconium content because the substantial amount of titanium present raises the level at which atmospheric contamination would occur. The titanium in the alloy appears to oxidize and enter into solid solution w th niobium oxide to form a protective oxide layer which inhibits further oxide penetration of the alloy. A two-layer surface oxide is thus formed, the inner layer appearing to be the more beneficial in'restricting additional oxide penetration.

Small amounts of var'ous other elements, usually less than 2% or 3%, can be included in the niobium base a1- loy without detracting from its mechanical properties. For example, up to 2% or 3% aluminum slightly improves the oxidation resistance of the alloy without reducing its hot strength. If the aluminum content is raised above this level, however, it embrittles the alloy by promoting the formation of an unstable miscrostructure. A small quantity of carbon serves to somewhat increase the strength and hot workability of the niobium base alloy, particularly when the zirconium content is relatively high. Large amounts of zirconium have a tendency to promote subsurface contaimination of the alloy by absorbing oxygen in solid solution. Under these circumstances, carbon may advantageously be added to the alloy and apparently combines with the zirconium to form zrconium carbide, thereby inhibiting this contamination. If more than 2% carbon is added, on the other hand, the room temperature ductility of the alloy is reduced to an undesirable extent.

When the zirconium content of the alloy is at a lower level, a small amount of boron, usually less than 0.01%, may be included in the alloy to further improve its hot workability to some degree. As indicated above, zircon um appears to absorb and/or react with oxygen at the alloy surface. Therefore, with the higher zirconium content alloy there is no need to add boron, which improves alloy ductility by apparently modifying grain boundary contaminants.

The above-described niobium base alloy has been prepared by alloying high purity elemental raw materials in an inert atmosphere of argon plus helium. It was found that the constitutents of the alloy may be added either simultaneously or successively. Melting was accomplished with a nonconsumable tungsten electrode electric arc. After solidification of the alloy, it was crushed to fine particle size and re-melted in the above-described manner and cast in ingot form. This ingot was enclosed in a container formed of an alloy of molybdenum plus 0.5% titanium, heated to approximately 2700 F., and extruded. Sound extrusions may be obtained with the nobium base alloy of this invention by direct extrusion of the cast ingot, the extruded alloy having a fibrous appearing cold worked structure. Subsequent further working has been readily accomplished by both swaging and rolling. This highly ductile alloy therefore can easily be made into sheet material.

Test bars were mach'ned from this extruded niobium base alloy bar stock and tested in an argon atmosphere at a temperature of 2000 F. under a load of 10,000 psi to evaluate the hot ductility of the alloy. The elongation chromium, l%'

. 4 scope of the invention is not to be limited thereby except as defined bythe following claims.

For example, a test bar of an alloy We claim:

1. A highly ductile, oxidation-resistant sheet metal article formed of an alloy comprising about 15% to 20% titanium, 9% to 11% molybdenum, 4% to 8% chromum, 1% to 5% zirconium and the balance substantially all niobium.

molybdenum and 1% zirconium was reduced 80% in area and had a 125% elongation atlrupture.

This alloy demonstrated a life of 16.1 hours to rupture under these 7 test conditions. We have also evaluated these alloys from the standpoint of oxidation resistance. For example, an alloy composed of about 60.5% niobium, titanium, 7.5 chromium, 10% molybdenum, 5% zirconium, 1% aluminum and 1% carbon had a total surface metalloss of less than 5 mils in thickness after 100 hours cyclic exposure in air at a temperature of 2000 F. The other alloys previously mentioned exh bited similar oxidation resistance. When subjectedto the foregoing stress-rupture test, a test-bar of the alloy containing 60.5% niobium'had a life of 13.7 hours with 68.7% elongation at rupture and a reduction in area of 54.4%

'In comparison, typical commercially available nickel base alloys, used in sheet form for similar purposes, have materially 'lessstrength than the alloy of this invention. For example, a commercial alloy consisting of 0.04% car bon, 0.5% manganese, 7% iron, 14.5% aluminum, 2.5% titanium,. 1% niobium plus tantalum I and the balance nickel, has a l0-hour stress-rupture life at l800 F. while under a load of only 4,000 p.s.i. Hence the strength of our alloy compares favorably with the strength of sheet materials heretofore employed in high temperature applications. The'new alloy thus'may be beneficially used for turbine engine components other than those which are subjected to extremely high stresses.

'The aforementioned data also show that the niobium' base alloy of the present invention possesses a combine tion of good oxidation resistance at extreme temperatures chromiun, 0.7%'

2. A sheet metal article characterized by outstanding ductility and oxidation resistance at elevated temperatures, said article being formed of analloy consisting essentially of about 15% to 20% titan'um,'9% to 11% molybdenum, 7% to 8% chromium, 3%'to.5% zirconium and the balance substantially all niobium.

3. A gas turbine engine sheet metal component characterized by outstanding room and high temperature ductil ty and oxidation resistance upon exposure to oxidizing gases at a. temperature of 2000 F., said sheet metal being formed of'an alloy comprising about 56% to 71% niobium, 15% to 20% titanium, 9% to 11% molybdenum, 4% to 8% chromium and 1% to 5% zirconium.

4. A sheet metal component for a gas turbine engine characterized by excellent ductility and oxidation resistance upon exposure to oxidizing gases at a temperature of 2000 F., said component being formed of an alloy consist'ng essentially of about 15 to 20% titanium, 9%

to 11% molybdenum, 4% to 8% chromium, 3% to 5% zirconium and the balance substantially all niobium.

5. A sheet metal article formedtrom a niobium base alloy having an outstanding combination of high temperature ductility, fabricablity and oxidation resistance at a temperature of 2000 F., said alloy consisting essentially of about 1 5% to 20% titanium, 9% to 11% molybdenum, 7% to 8% chromium, 1% to 5% zirconium, aluminum not-in excess of approximately 3%,

a carbon not in excess of approximately 2%, boron not in and outstanding ductility. The alloy has a melting point materially in excess of 3000. F., the desired minimum temperature hereinbefore mentioned.

While our invention has been described by means of certain specific examples, it is to be understood that the excess of approximately 0.01% and the balance substan tially all niobium. .1

References Cited in the file of this patent UNITED STATES PATENTS 

1. A HIGHLY DUCTILE, OXIDATION-RESISTANT SHEET METAL ARTICLE FORMED OF AN ALLOY COMPRISING ABOUT 15% TO 20% TITANIUM, 9% TO 11% MOLYBDENUM, 4% TO 8% CHROMUM. 1% TO 5% ZIRCOMIUM AND THE BALANCE SUBSTANTIALLY ALL NIOBIUM. 